Well, it seems "Hortman" identified one of the most common reason for white spots in Tomatoes. I'd like to add to his answer with common reasons for white spots on tomatoes here on the West Coast.

Powdery mildew occurs in most soft leaf plants like tomatos. The fungus is carried by spores that are carried by wind to plants. Watering too late in the day, poor drainage, and lack of sun increase disease development.

To treat this mildew use multiple applications of non-systemic fungucides like Danconil,Lilly Miller Sulfur Dust, or Safer Garden Fungicide.

Leaf Spot can also be a problem. This disease begins in the leaves of tomatoes, starting with the oldest leaves. Both light and dark spots can be seen on both the top and bottom surfaces of the leaves. The spots enlarge and turn a grayish brown. Plants affected by gray leaf spot disease should be treated or removed from the garden to avoid affecting the other plants.

Fungicides like the ones mentioned above are effective if applied on a every week for at least 3-4 weeks. If you discover you have Leafspot, do not plant in the same place the following season to prevent reintroduction of fungus. The following year should be fine.

Another common pest is the Leafminers. These bugs are regarded as pests by farmers and gardeners because they cause damage to plants. They can be difficult to control with common insecticide sprays because they are protected inside the plant's leaves.

Spraying the infected plants with an non systemic insecticide like Bayer Advanced, Bonide,Ortho Fruit and Vegetable, or Monterey Garden are all effective treatments.

The most important part of dealing with garden problems is addressing the problem quickly rather than putting it off till it's too late. Please feel free to ask more questions you might have concerning your tomatoes.